cubicweb #3463112 [schema] fix composite deletion handling [validation pending]

As of today, the deletion chain is implemented by an hook on composite relations.

Unfortunately the related ("component") entities are wiped even if the composite entity is not deleted, iow: composite relation deletion entail component deletion.

This surprised end-users more than once.

priorityimportant
typebug
done in3.19.0
load0.100
load left0.000
closed by#be001628edad [schema] fix composite deletion handling
patch[schema] add composite handling helpers on EntitySchema (related to #3463112) [applied][schema] fix composite deletion handling [applied]